The 80mm/f5.6 has 6 elements, regardless if it is an 'N' (newer plastic with lighted f stops) or non-'N' (metal, non-lighted f stops). The 75mm has 4 elements.
My 80/5.6 is the old metal one. It is an early one and is fine. The 50mm/f2.8 el-nikkor that I also one is an old one as well. I do jump up and down over this particular one.
I think my 80/5.6 is just okay. The one on that ebay auction your specified is newer (much newer) and I wouldn't hesitate on it.
Other than that, the best way to figure out of it's got 6 elements.. the number of reflections if you shine a pen light into it, reading the manufacturers datasheet.. asking here on apug